Method and apparatus for drilling in deep water

ABSTRACT

The specification discloses apparatus and methods for conducting operations in an underwater well from a floating vessel without use of a riser pipe. The system includes an elongated member having a hollow longitudinal bore extending from its upper end to its lower end and sized to permit a drill string to pass therethrough. A seal means is situated near the upper end of the hollow member for sealing the drill string within its bore. This seal means is adapted to permit the drill string to rotate and to translate vertically with respect to the member. The hollow tubular member is of adequate length to permit a span of drill string to extend downward therethrough beneath said seal having a weight sufficient to overcome the differential pressure acting on the drill string as a result of the difference between well pressure and hydrostatic head. Means are provided near the lower end of the elongated member for remotely connecting it to a subsea installation provided with valve means for opening and closing the orifice of the well.

United States Patent [19] Lloyd et al.

[ METHOD AND APPARATUS FOR DRILLING 1N DEEP WATER [73] Assignee: Esso Production Research Company, I

Houston, Tex.

[22] Filed: Dec. 5, 1972 [21] Appl. No.: 312,415

[52] U.S. Cl. 166/.5, 175/7 3,196,958 7/1965 Travers et al 175/7- 3,554,277 1/1971 Bauer et al. 175/7 3,621,912 11/1971 Woody et al. 166/.5

3,722,584 3/1973 Nelson 166/.5

[451 July 23, 1974 Primary Examiner-Frank L. Abbott Assistant ExaminerRichard E. Favreau Attorney, Agent, or Firm-James E. Gilchrist 57] ABSTRACT The specification discloses apparatus and methods for conducting operations in an underwater well from a floating vessel without use of a riser pipe. The system includesan elongated member having a hollow longitudinal bore extending from its upper end to its lower end and sized to permit a drill string to pass there through. A seal means is situated near the upper end of the hollow member for sealing the drill string within its bore. This seal means is adapted to permit the drill string to rotate and to translate vertically with respect to the member. The hollow tubular member is of adequate length to permit a span of drill string to extend downward therethrough beneath said seal having a weight sufficient to overcome the differential pressure acting on the drill string as a result of the difference between well pressure and hydrostatic head. Means are provided near the lower end of the elongated member for remotely connecting it to a subsea installation provided with valve means for opening and closing the orifice of the well.

5 Claims, 3 prawing Figures PATENTED 3.825.065

' sum 1 or 2 FIG. I

1 V APPARATUS FOR DRILLING IN DEEP WATER BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention is directed to an improved systern for conducting operations in a subsea well from a floating vessel which includes a retrievable assembly adapted to be positioned atop a subsea installation to permit well operations to be conducted without use of a riser-pipe.

2. Description of the Prior Art In recent years the search for offshore deposits of crude oil and'natural gas has been extended into increasingly deeper waters offshore the continental land masses. With the increased water depths drilling operations have more frequently beenconducted from floating vessels since economic considerations militate against the use of bottom-founded platforms. In these operations the drill rig and associated equipment are METHOD AND positioned aboard a floating-vessel stationed over the wellsite. Thedrill string extends from the vessel to a wellhead situated on the floor of the body of'water,

normally through a riserpipe which interconnects the vessel and wellhead and serves to guide the drill string into the wellhead and to conduct the returningdrilling fluid back to the vessel. I

As water depths and drilling depths increase, requiring longer riser pipes and more dense drilling fluids, the stresses within the riser pipe increase substantially. The effects of these factors have been mitigated in the past by applying tension to the'riser pipe near the upper end and in some instances byattaching buoyant material to its exterior. While these techniques have proven effective in water depths up to 1500 ft.or more, they too are depth limited, and depths will ultimately be reached head requires a downward force of sufficient magnitude to overcome the difierential pressure. This force must be greater than the product of the cross-sectional area of the drill string and the differential pressure. Unfortunately, it will frequently be of a magnitude such that the length of drill string required to generate the necessary gravitational force is so long that it will be unstable and tend to buckle of its own weight.

{SUMMARY or THE INVENTION The difficulties outlined above are greatly alleviated by the method and apparatus of the present invention. In accordance with the invention, apparatus is provided which permits the introduction of the drill string into a subsea wellhead in the face of the differential pressures existing when deep water drilling operations are conducted without use of a marine riser.

The apparatus of the invention includes an elongated member having a hollow longitudinal bore which extends from the upper end to thelower end of the member and is sized to permit the drill string and drill bit to pass therethrough. A means is situated near the upper end of the tubular member for sealing the drill string within the bore. This means is adapted to permit rota- 6 tion and vertical translation of the drill string with renecting' its lower end to a wellhead or similar subsea installation provided with a valve means for opening and gclosingthe well. The length of the hollow member where the expense of tensioning theriser is prohibitive 'string was guided to and from the subsea well by two or more flexible guide lines extending between the subsea wellhead to the vessel. Such systems did not see widespread use because in shallow water a riser pipe can be used quite conveniently and will at the same time eliminate the need for utilizing a subsea rotating seal assembly, a relatively complex device requiring frequent maintenance.

The problems experienced with the large diameter drilling risers indeep water, however, overshadow the difficulties involved in utilizing apparatus provided with a rotating seal assembly in a relatively inaccessible subsea location. Operations without a riser pipe in deep water are, however, characterized byproblems not present inshallow water.. A significant problem. has been found to occur when. deep formations are being drilled, requiring the drilling fluid to have adensity greater than seawater. In such circumstances, the mud column will exert a pressure at the wellhead substantially in excess of that of the surrounding sea water. As a result, to extend the drill string into the subsea wellshould be adequate to permit a span of drill string to ,extend therewithin between the seal means and the valve means which will have a weight sufficient to overcome the differential pressure between the column of drilling fluid and the hydrostatic head.

In accordance with the method of the invention, an elongated hollow chamber is positioned atop the subsea well. A span of drill string is introduced into the chamber while the valve means on the subsea installation is closed. The span should be of adequate length to develop a gravitational force of sufficient magnitude to permit introduction of the drill string into the well without buckling the string. The upper end of the chamber is sealed around the drill string to provide a pressure tight seal which permits the string to rotate and to move vertically. The valve means is then opened to permit equalization of pressure between the chamber and the well. The drill string is lowered to permit the span to descend into the well and to apply a vertical force, as required, to the upper end of the span so that the total downwardly directed force acting on the span is sufficient to overcome the upward force caused by the difference in pressure between the well and the surrounding body of water. The lower end of the drill string is positioned on bottom and well operations are conducted in the normal fashion with the seal at the upper end of the assembly serving to contain the well FIG. 1. is an elevation view partly in section depicting a floating drilling system wherein the apparatus of the present invention is situated atop a blowout preventer stack mounted ona subsea wellhead. v

FIG. 2 is an enlarged elevation view partly in section depicting the apparatus of the present invention shown in FIG. 1.

FIG. 3 is a schematic elevation view of the apparatus of the invention depicting the forces acting on the drill string as it is introduced into the bore of the hollow tubular member.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Y With reference to FIG. 1 of the drawings, a subsea wellhead 11 is shown situated adjacent the floor 15 of a body of water. In accordance with normal operating practice itis positioned atop a string of casing 13 which is cemented withinia borehole extending downwardly into the substrata-A'blowout preventer stack, designated generally by numeral 17, is situated atop the subsea wellhead. The stack shown is typical of those in commercial use and includes three ram-type preventers numbered 19 and a single bag-type preventer designated by numeral 21 A short section of the bundle of lines which normally interconnect the blowout preventer stackand 'the' floating vessel situated at the waters surface is shown in cutaway and-identified by numeral 23. The bundle of lines permits remote operation of the various control functions of the BOP and may, for example, include hydraulic fluid supply lines, control lines, choke and kill lines, and in some instances a gas lift line. In accordance with the invention a small diameter return line for conducting drilling fluid back to the vessel may conveniently be included in this bundle and is designatedby numeral 26in the drawing. Subsea valve 24 is depicted in the drawings as controlling flow through the return line. The return line may be a flexible hose or, alternatively, it maybe made up of rigid lengths of pipe having flexible connectors therebetween. Other typical subsea components shown include a pair of guide posts 25 together with guide lines 27 connected to the wellhead. The guide lines extend upwardly towards vessel 28 situated at the surface of the water and serve to guide equipment as it is raised and lowered between the vessel and the wellhead.

Shown positioned atop the blowout preventer stack is one embodiment of the apparatus of the present invention. As shown, the apparatus includes an elongated member, designated by numeral 31, having a hollow longitudinal bore extending from its upper end to its lower end. The elongated member is preferably a tubular member and is sized to permit a drill string and drill bit to pass therethrough. The apparatus includes a pair of guide arms 29 provided with guide sleeves 30 at their outer extremities to connect the apparatus to the guidelines'andthereby facilitate moving it between the vessel and the BOP stack and aligning it for connection to the stack. As shown in FIGS. 1 and 2 of the drawing, the elongated member may be fabricated from a series of pipe joints having threaded end connections 33. Other means of constructing the tubular member will readily suggest themselves to those skilled in the art, it being apparent that the hollow member must withstand the differential pressure between the well and the sur- 4 rounding body of water to perform its intended function.

Means situated near the upper end of the hollow member engages and seals the drill string within the bore of the tubular member. This seal means is adapted to permit the drill string torotate with respect to the elongated member while maintaining the seal. It further permits the drill string to move vertically therethrough to enable the drill string to be raised or lowered without breaking the seal. The apparatus for providing this seal shown in FIG. 1 and FIG. 2 is comprised of a bag preventer 35 affixed to the upper endlof the tubular member and provided with a rotating head. Such preventers are commercially available and permit rotation and vertical movement of the drill string without loss of well pressure. Padeyes, such as those designated by numeral 37, hooks, or the like,.may be affixed to the upper end of the tubular member to facilitate hanging-it from the drilling vessel when it is situated at the surface of the body of water, e.g., when it is withdrawn to change out the annular seal in the rotating'head. r

A means is situated near the lower end of elongated tubular member 31 for remotely connecting and dis-v,

connecting the assembly to a-subseainstallation associated with a subsea well and provided with valve preventer stack. However, the apparatus of the inventioncan. be directly connected to the wellhead, e. g., when it is desired to reenter a producing well. As shown in FIG. -1, the connector assembly may consist of a remotely actuated hydraulic connector such as that de picted by numeral 39. Such connectors are in commercial use in floating drilling operations and are normally employed to connect and disconnect the lower end of the drilling riser from the blowout preventer stack.

The distance between the upper seal means and the valve means should be adequate to permit a sufficient length of drill pipe to extend beneath the seal and through the bore to have a weight such that its gravita-. vtional force will overcome the differential pressure act- The provision of buoyancy in this fashion has the advantage of reducing the effective weight of the assembly and, thus, reducing loads during its-running and withdrawal. The buoyant member is particularly advantageous when the hollow tubular member has an extended length as it acts to provide lateral support. In such instances, sufficient buoyancy can be provided to make the assembly self-standing, thereby eliminating any tendency of the member to buckle.

A flexible joint such as the ball joint designated by numeral 43 may advantageously be provided at the lower end of the assembly. The ball joint permits elongated member 31 to pivot in response to imposition of external. loads, thereby relieving stresses caused by bending moments acting on the assembly. It has the further advantage of protecting the wellhead and'BOP stack from moments that otherwise might be transferred to them by the pressure chamber assembly.

Turning to FIG. 3 of the drawing, a schematic elevation of the apparatus of the invention is shown which depicts the forces actingonthe drillstring as it is introduced into the retrievable subsea assembly. The upper portion of the hollow tubular member of the present invention is shown and is designated by numeral 31. The upper seal assembly, which permits rotation and vertical movement of the drill string within the tubular member while maintaining an annular seal around it,'is designated by numeral 35. A span of drill string, desig nated bynumeral 45, is shown extending within the inner bore of the tubular member and having its upper end sealedtherein by seal35. The length of the section of drillstring extending below the upper seal is designated in FIG. 2 as L while its cross-sectional area is designated as A The pre'ssurewithin the well, P will normally be that exerted by the column of the drilling fluid extending from the tubular memberto the water surface through the return line. The pressure of the surrounding water 'P;, will be equal to the product of the applicable water pressuregradient and the water depth of the tubular member. The span of drill string has a weight per unit length designated as Wg, which corresponds ,to its weightin the fluid within the tubular member. i

Considering the. forces acting within the system shown in. FIG. 3, and assuming the drill string is not in compression and the tubular member contains drilling fluid, it will benoted that the downward force exerted by the span of drill string extending below seal member 35 will be equal to the length of the span times its weight per unit length in drilling fluid, or (L,,,,) X (W,;,). The force acting upwardly on the drill string and tending to prevent it from descending within the bore will be the difference in pressure between .well pressure after pressure equalization between the chamber and the well without imposition of any external force. If a shorter length of drill string extends within'the assembly, the force exerted by the differential pressure between the well and the hydrostatic head will exceed the gravitational force and be sufficient to prevent the drill string from descending, absent the imposition of vertical force on the drill string above the seal. While the length of the tubular member will be governed by the above variables in each instance, it will normally be at least 100 feet long. v

By way of example, if the upper seal assembly is to be situated at a depth of 2,000 feet in sea water having a specific weight of 8.44 lb/gal, the hydrostatic head (P,,) exerted by the seawater will be about 875 psi. If a mud having a density of 12.0 lb/gal. is used, the pressure exerted by a 2000 foot column of mud, i.e., the well pressure (P,,,), will be about 1248 psi, Assuming that the drill string is comprised of drill collars having a nominal OD of 7%inches and ID of 2% inches and'a weight per unit length (W,,,) is 12.0 lb/gal. drilling fluid of 114 lb/ft, the cross-sectional area of the drill string (A will be 47.17 sq. in. The total upward force caused by the pressure differential will be 47.17 in X (1248 psi 875 psi) or 15,594 lb. The length of drill string requiredwill thus be 15,594 lb 141 lb/ft. or about 137 feet.

Normal operating practice would be to provide a tubular'member having a length at least sufficient to position the upper seal assembly a sufficient distance above the valve means in the BOP so that the length of drill string dictated by the above equation, i.e. 137 ft. in the example, can extend therebetween. This has the advantage of permitting introduction of the drill string into the well without placing the portion of the string immediately above the tubular member in compression. It has the further advantage of permitting the requisite length of the drill string to be positioned within the assembly when it is initially lowered and connected to the installation. It will frequently be convenient to provide a tubular member having a longer length than initially required in anticipation of encountering increased well pressure as drilling depth increases. It should'be noted,

however, that a somewhat shorter length of the tubular member can be tolerated if a shorter span of drill pipe is introduced into the assembly and the portion of the drill string just above the tubular member is placed-in compression to provide the additional downward force necessary to promote entry of the drill string into the wellThe amount of additional force that can be generated in this fashion without damaging the pipe is limited and willbe dictatedby the magnitude that the drill string can withstand without buckling. In any event the total force downward must exceed that generated by the differential pressure between the well and the hydrostatic head in order for the drill string to be introduced into the well.

Once subsea wellhead 11 is attached to surface casing 13 which is cemented within the borehole and the blowout preventer stack l7is positioned atop the wellhead, the apparatus of the present invention may be lowered into place. The apparatus is normally assembled on the drilling vessel and then installed subsea. It will frequently be convenient to assembly the tool section'by section and hang it from the vessel by padeyes like those depicted by numeral 37 of the drawing until it is completely assembled. With guide arms 29 and sleeves 30 connecting the assembly to guide lines 27 which direct its descent, the ass'embly'is lowered into position, usually by means of the drill string. Running the assembly on the drill string normally involves use of a running tool of the type which includes a J-slot and is known in the art of offshore drilling. Tubular member 31 and associated elements of the assembly will be lowered towards bottom until the lower end of the assembly comes into contact with the upper end of the BOP stack. Hydraulic connector 39 isthen actuated to connect the lower end of the assembly to the upper end of the BOP. stack.

As noted above, it will be convenient to lower the assembly with a sufficient length of drill string already within itsbore to overcome the differential pressure exerted by the well fluid. To prevent loss of drilling fluid and any contamination of the surrounding sea water, the drill string and hollow tubular assembly can be run containing sea water. With the hydraulic connector engaged, seal assembly 35 is actuated so that the drill string is slidably and rotatably sealed within the tubular member. Preferably the subsea valve on the return line is then opened and, if the system contains water, drilling fluid is circulated down the drill string and out the return line until the water isdisplaced from the drill string and the hollow tubular member and circulation of drilling fluid is established. The valve means on the subsea installation, e.g., bag prevent'er 21 atop the BOP ,stack, is then opened to permit equalization of pressure between the Welland the bore of tubular member 31. 'If the system is run filled with water and the return line and associated subsea valve are connected to the BOP stack beneath the valve used to close in the well, the

valve is opened'and pressure equalization is achieved prior to circulating out the water. Because of the difference in pressure gradient between mud and water it may be desirable in suchv circumstances to maintain-a back pressure on-the return line so as: to control the pressure exerted by -the column of fluid downhole. With upper seal assembly 35 holding the well under control, the runningtool is disengagedJThis is typically accomplished by slacking off and then-rotating the drill drill string into the well without buckling the string; 5

. c. sealing the upper end of said chamber around said drill stringto provide a pressuretight seal while Y permitting the string torotate and move vertically;

d. opening said valve means to equalize pressure within said chamber and said well; and, thereafter e. lowering thedrill string to permit .said span to de- I scend into said Welland to apply a vertical force, as required, to the'upper end of said span so that the total of the downwardly directed forces acting on said span is greater thanthe upward force 3 further including the step of moving said hollow chamber between said vessel andsaid subsea well while guiding said chamber alongflexible guide means extending between said vessel and said well.

5. -In an apparatus for conducting operations from a floating vessel wherein a drill string-is to be extended into awell situated onia submerged bottom and provided with valve means near the upper end thereof and drilling fluid is returned to the vessel through a flexible return conduit independent of the drill string and the operations are of the type wherein the pressure differof the tubular member or above the B0? or other valve means used. The valve means, normally a blind ramtype preventer 19, is then closed and will serve to provide well control. The drilling fluid within the bore of the drill string and the tubular member is then normally circulated out with sea water to relieve pressure and to prevent contamination of'the surrounding body of water. 'As indicated above, this may be carried out by pumping sea water down the drill string and back through the return line. The running tool is then reconnected by elevating the drill string to engage it and then rotating the drill string clockwise. Hydraulic connector 39 is disconnected from the BOP stack to permit withdrawal of the assembly to the surface. The assembly is then raised by means of the drill string to the vessel where it may again be hung in the moonpool to permit inspection and maintenance of upper seal assembly 35. The. drill string rnay then be withdrawnfrorn the bore of the assembly and raised to the level of the to change the bit or otherwise alter the equipment.

What is'claimed is: 7

' 1. A method of conducting operations in a well situated on the floor of a body of water from a vessel floating on the water surface wherein said well-is provided with valve means for opening and closing its bore comprising:

a. positioning an elongated hollow chamber atop said subsea well;

b. introducing a span of drill string into said chamber with said valve means closed, said span having a length sufficient to develop a gravitational force of sufficient magnitude to permit introduction of the ential between the hydrostatic head of the drilling fluid and that of the surrounding body of water taken at the valvemeans is sufficient to create an upward force that will cause the drill string to buckle if it is attempted to be introduced into the well, the improvement comprising: r

a. an elongated member having a hollow longitudinal bore extending from the upper end to the lower end thereof and sized to permit the drill string to extend therethrou'gh;

b. means situated near the upper end of said member for sealing the drill string within said bore, said sealing means permitting said drill string to rotate and to translate .vertically with respect to said elongated member while maintaining a'seal therebe- 4 tween;

c. means situated .near the lower end of said elongated member for remotely connecting and disconnecting the lower end of said elongated member to said well in a manner to permit fluid communication between the bore of said elongated member and, the bore of said. well through said valve means; and 1 said elongated member being at least long enough to permit a. span of drill string to extend downwardly therethrough between said seal means and said valve means which span will exert a gravitational force sufficient-to overcome the difference in pressure between well pressure and that of the hydrostatic head of the surrounding body of water and permit said drill string to be introduced into the well without buckling said drill string. 

1. A method of conducting operations in a well situated on the floor of a body of water from a vessel floating on the water surface wherein said well is provided with valve means for opening and closing its bore comprising: a. positioning an elongated hollow chamber atop said subsea well; b. introducing a span of drill string into said chamber with said valve means closed, said span having a length sufficient to develop a gravitational force of sufficient magnitude to permit introduction of the drill string into the well without buckling the string; c. sealing the upper end of said chamber around said drill string to provide a pressure tight seal while permitting the string to rotate and move vertically; d. opening said valve means to equalize pressure within said chamber and said well; and, thereafter e. lowering the drill string to permit said span to descend into said well and to apply a vertical force, as required, to the upper end of said span so that the total of the downwardly directed forces acting on said span is greater than the upward force caused by the difference in pressure between the well and the surrounding body of water.
 2. The method defined by claim 1 further including the step of buoyantly supporting said elongated hollow chamber near the upper end thereof.
 3. The method defined by claim 2 further including the step of flexibly connecting said elongated hollow chamber to said subsea well.
 4. The method defined by claim 3 further including the step of moving said hollow chamber between said vessel and said subsea well while guiding said chamber along flexible guide means extending between said vessel and said well.
 5. In an apparatus for conducting operations from a floating vessel wherein a drill string is to be extended into a well situated on a submerged bottom and provided with valve means near the upper end thereof and drilling fluid is returned to the vessel through a flexible return conduit independent of the drill string and the operations are of the type wherein the pressure differential between the hydrostatic head of the drilling fluid and that oF the surrounding body of water taken at the valve means is sufficient to create an upward force that will cause the drill string to buckle if it is attempted to be introduced into the well, the improvement comprising: a. an elongated member having a hollow longitudinal bore extending from the upper end to the lower end thereof and sized to permit the drill string to extend therethrough; b. means situated near the upper end of said member for sealing the drill string within said bore, said sealing means permitting said drill string to rotate and to translate vertically with respect to said elongated member while maintaining a seal therebetween; c. means situated near the lower end of said elongated member for remotely connecting and disconnecting the lower end of said elongated member to said well in a manner to permit fluid communication between the bore of said elongated member and the bore of said well through said valve means; and d. said elongated member being at least long enough to permit a span of drill string to extend downwardly therethrough between said seal means and said valve means which span will exert a gravitational force sufficient to overcome the difference in pressure between well pressure and that of the hydrostatic head of the surrounding body of water and permit said drill string to be introduced into the well without buckling said drill string. 